On page 615 of the article "Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?" by Ulrich Zimmermann, Heike Schneider, Lars H. Wegner, and Axel Haase (New Phytologist 162: 575–615), the authors state:
"The experiment of Holbrook
et al. (1995) is faced with the same shortcomings as the various ‘vulnerability’ methods. They used an excised stem segment with a single leaf at its midpoint, and mounted the midpoint of the stem on the rotating axis of a centrifuge-like set-up placed in a closed chamber. After centrifugation the authors removed the leaf and determined the balancing pressure value. In the light of the discussion in section III.3 tension in the xylem should be released instantaneously upon cutting (provided that no breakage of the water columns had occurred). Nevertheless, the authors found a 1 : 1 relationship between the (relative) pressure calculated from the centrifugation force and the balancing pressure. A possible explanation for the 1 : 1 correlation is that water was shifted into the periphery of the leaf (including the intercellular spaces) during centrifugation. Then, Newton’s law that action has to equal reaction requires that the same force is needed to push water back from the tissue into the xylem. The problem for interpreting the data is the control experiment of Holbrook et al. (1995). It is obvious from the sketch in Fig. 1 of their paper that the control leaf, being not attached to the branch, was spun simultaneously, but not fixed close to the rotor axis of the centrifuge. In this case, the leaf is pressed against the wall of the chamber during rotation and, in turn, the centrifugal forces may act in a different way on the water in the control leaf compared to the fixed leaf. This could explain why the Pb-values of the control leaves were significantly lower than those of the attached leaves by 0.2 to 0.4 MPa. A correlation with the calculated rotational tension can also not be expected under these conditions. However, the finding that the Pb-values of the control leaves were significantly higher than the values of untreated leaves (0.05 MPa relative to atmosphere) evidences clearly that the control experiments were not properly designed."Summary comment:
The critique given by Zimmermann et al. (2004) of Holbrook et al. (1995) is fundamentally flawed. Figure 1 of Holbrook
Detailed explanation:
To propose their "flaw", Zimmermann et al. ignore the data in footnote 13 and instead propose an imaginary scenario they believe will produce the result they desire. Taking Zimmerman et al.'s statements at face value, the forces Zimmermann et al. describe shifting the water to "…the periphery of the leaf (including the intercellular spaces) during centrifugation" range from factors of 225 to 2500 too small to account for the 1:1 relationship of the Scholander reading to the "calculated rotational tension" presented in Figure 2 of Holbrook et al. As clearly stated in footnote 13 of Holbrook et al., the lengths of the Cercis occidentalis branches ranged from 30-100cm, thus the R’s used to calculate the "calculated rotational tension" (x-axis of Figure 2) ranged from 15-50cm while the maximum extension from the rotation axis any part of a leaf even using Zimmermann et al.'s incorrect description of the Cercis occidentalis leaves is 1cm, limited by the chamber size clearly stated in footnote 13. The induced tension in the water column at the rotation center is T=0.5sw2R2, where s is the density of water, w the angular velocity, and R is the distance from the axis of rotation to the end of the water column, which Holbrook et al.'s Figure 2 x-axis, was half the branch length. Zimmerman’s imaginary scenario results in a discrepancy of a factor of 152:1 to 502:1 (225:1 - 2500:1 ) from the observed 1:1 ratio. In other words, since the x-axis of Figure 2 of Holbrooket al. is a calculated value usingR's of 15-50cm, and the induced tensions anywhere there is an intact water column depend on R2, leaves with a maximum moment arm (R) of 1cm cannot have tensions induced between their centers and peripheries sufficient to come anywhere near the calculated values where R ranged from 15 to 50cm. (The 1:1 line of Figure 2 of Holbrook et al.) Zimmermann et al.'s (erroneous) scenario could, based on simple physics any High School physics student would understand, induce tensions 1/225th to 1/2500th of those needed to produce the 1:1 observations of Holbrook et al. Thus Zimmermann et al.'s imaginary "possible explanation" results in tensions 2 -3 orders of magnitude too small to produce the observed 1:1 ratio of Figure 2. Had Zimmermann et al.'s imaginary scenario occurred in the test leaves, then the test leaves would be indistinguishable from the control leaves.
In addition, had Zimmermann et al. looked up what Cercis occidentalis leaves look like, it would have been obvious that both the test and control leaves had to be wrapped together around the inner chamber walls, thus experiencing exactly the same forces and thus precluding their proposed scenario from occurring even with its deficiencies. In other words, the experiment of Holbrook et al. is even more robust than it may appear to an inattentive reader.
Given the overall tenor of Zimmermann et al., we will leave it to the reader to speculate as to why Zimmermann et al. would advance the critique on Holbrook et al. that they did, hidden in the second appendix their paper and without presenting any estimates for the magnitude of the effect they proposed despite such estimates being in a realm that any High School physics student could easily calculate.
Holbrook et al. (1995) - "Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique", N. Michele Holbrook, Michael J. Burns, and Christopher B. Field, Science 270, 1193 (1995)
Zimmermann et al. (2004) -"Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?", Ulrich Zimmermann, Heike Schneider, Lars H. Wegner, and Axel Haase New Phytologist 162: 575 (2004).
Editor Ian Woodward of New Phytologist response to the New Phytologist 163: 451–452 (2004) protest letter of Angeles et al.. (New Phytologist 163: 4532 (2004))
Other related links:
Hervé Cochard's Zimmermann Comment Page